Author Topic: Designing steam engines-oscillators specifically  (Read 4352 times)

Offline chucketn

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Designing steam engines-oscillators specifically
« on: June 25, 2018, 09:42:04 PM »
I am studying and experimenting with designing my own steam engine. I’’m starting with Oscillators. Specifically a double cylinder, double acting.
A member on another forum posted a link to an Ocillator simulator program, available here, that simulates simple single and double acting oscillator steam engines. It’s a neat program, and I learned a lot from it.
One thing that I haven’t been able to figure out is whether the piston should cover, not cover, or cover half the port openings at the top or bottom of it’s stroke. I assume(and you know what that does) it has something to do with timing and ‘cutoff’ in engines run on steam, but I don’t yet have a boiler, and will be running mine on compressed air.
The above mentioned simulator shows  the piston covering half the port in one engine and not covering the ports I another. Can someone explain the difference or wherewithal of the piston covering, half covering, or not covering the port?

Offline crueby

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Re: Designing steam engines-oscillators specifically
« Reply #1 on: June 25, 2018, 09:52:53 PM »
Im the ones I've built the piston did not cover the ports at all, just depended on the motion of the cylinder across the plate to connect/disconnect the holes as it wobbles.
Interesting question....   :thinking:

Offline Brian Rupnow

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Re: Designing steam engines-oscillators specifically
« Reply #2 on: June 25, 2018, 10:03:12 PM »
On the oscillating engine that I just posted a thread on building, the piston is 0.433" thick and it completely covers the ports. The ports in the cylinder which align with the ports in the frame underneath are both out near the end of the piston travel and are covered completely by the piston.

Offline Gas_mantle

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Re: Designing steam engines-oscillators specifically
« Reply #3 on: June 25, 2018, 10:42:20 PM »
My wobblers have ports that aren't covered.

Offline zeeprogrammer

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Re: Designing steam engines-oscillators specifically
« Reply #4 on: June 26, 2018, 12:29:19 AM »
Not knowing much about such things...it seems to me that if a piston covers a port completely...then steam/air can't get in to push it back.
Unless the idea is that the piston will pull back a bit due to inertia of the system.

Again..not knowing much about such things...I would have thought that the piston would never cover a port. Just seems cleaner to me.

I'll be interesting in seeing more in this thread.
Carl (aka Zee) Will sometimes respond to 'hey' but never 'hey you'.
"To work. To work."
Zee-Another Thread Trasher.

Offline MJM460

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Re: Designing steam engines-oscillators specifically
« Reply #5 on: June 26, 2018, 03:53:07 AM »
Hi Chuck, there is nothing like designing your own to show you which dimensions are critical to make it all fit together.  It's a fun challenge, and an oscillator is a great place to start.

First, the piston covering the port or not, is only about steam flow, not timing.  Timing is controlled purely by the oscillation of the cylinder alternatively uncovering the inlet and exhaust ports at the port face between the cylinder and stand.

When the piston is at top dead centre, the cylinder port should lie between the inlet and outlet ports on the stand, and both are closed.  When the oscillation moves the cylinder port towards the inlet port, the fluid inlet is controlled by the gradual opening of the overlapping sections of the cylinder and stand ports.  The port opens to a maximum at the extreme of the oscillation then starts closing again to be totally closed at bottom dead centre.  Similar applies to the bottom cylinder port on the double acting engine.

The opening of the port inlet port turns out to be not too critical, but the opening on the exhaust stroke needs to be as quick as possible as the highest exhaust flow required is immediately the port starts to open, just after bottom dead centre.  As the cylinder ports are alternatively inlet and exhaust, the exhaust requirements govern the design of the cylinder ports.

It is desirable to minimise the clearance volume at the end of the stroke, so this is where the piston can cover the port, just when you don't want it.

I make the cylinder about three mm longer than required, and machine a spigot on the inside of the head to fill the volume down to the required clearance at each end.  This spigot locates the head on the cylinder, and is necessary on the bottom head of a double acting engine any way, so I include this detail on the top head as well.  Then I can machine away the little half circle of the spigot that covers the passage from the cylinder block, leaving an opening with the full flow area.

The inlet port is on the same side of the stand for both top and bottom inlets, so with careful design you can connect the two ports with a hole drilled top to bottom, and either use the end of the hole as the engine inlet, or plug it if you prefer a side entry.  Similarly for the exhaust side.

I prefer to plug the vertical  hole, and cross drill again at the bottom and design a valve into the stand connecting the inlet and exhaust ports when open.  This can be opened to allow a steam flow before the engine starts, in order to warm up the block.

Basically no difference in porting for steam or air, but all your fabrications and connections need to have strength at steam temperature, so use of epoxy or adhesive is more limited for steam.  And you need to make provision for handling a bit of condensate on startup.  And you need a different type of oiler for air.

I hope that answers the question.  Looking forward to following your progress.

MJM460

« Last Edit: June 26, 2018, 03:57:13 AM by MJM460 »
The more I learn, the more I find that I still have to learn!

Offline Jasonb

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Re: Designing steam engines-oscillators specifically
« Reply #6 on: June 26, 2018, 07:23:48 AM »
As you will start to see the ports opening with maybe just 1 degree of crank movement past dead ctr then if the piston were fully covering the port you would not be allowing that air/steam in and the exhaust gasses would be getting compressed at the other side  of thr cylinder taking up energy from the incomming.

For efficiency a minimal void is desirable at the ends of the piston so the design of the passage from port to cylinder can be used to help get the air/steam right to the end. A simple wobbler may be drilled straight through from port face to cylinder but more sophisticated ones have passages much like any slide valve steam engine.

Same question came up teh other day about Muncasters oscillator so I sketched it out quickly to show how soon the port opens.

<a href="https://www.youtube.com/watch?v=9WhoQZmlcIs" target="_blank">http://www.youtube.com/watch?v=9WhoQZmlcIs</a>


This is the one I built over Christmas, you can see how the passages take the air right to the end of the cylinder



And where the ports are much nearer the pivot




Offline chucketn

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Re: Designing steam engines-oscillators specifically
« Reply #7 on: June 26, 2018, 11:43:24 AM »
Wow! Thanks for the responses! All this info will greatly aid my understanding of the finer points of Oscillators. Not to worry, I have lots more questions...

Offline Brian Rupnow

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Re: Designing steam engines-oscillators specifically
« Reply #8 on: June 26, 2018, 01:04:55 PM »
Chucketn--My answer may have been a bit misleading--When viewed from directly above the cylinder does hide the ports. However, there is a bit more to it than that. This side view gives a much better understanding of where the port passages are in the cylinder compared to piston position.---Brian

Offline chucketn

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Re: Designing steam engines-oscillators specifically
« Reply #9 on: June 26, 2018, 03:17:45 PM »
Thanks Brian, that picture does clarify your port arrangement. I'm not trying to steal your thunder on selling the plans to your very nice wobbler, I wish you great success with that venture :cheers:. I just wish to understand the design process better. I think I will accomplish that by designing my own. :NotWorthy:

Offline Tennessee Whiskey

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Re: Designing steam engines-oscillators specifically
« Reply #10 on: June 26, 2018, 03:27:53 PM »
Chuck, Mr. Pete222 does some good YouTube videos on wobblers. They really show the porting wel.

Cletus

Offline chucketn

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Re: Designing steam engines-oscillators specifically
« Reply #11 on: June 27, 2018, 12:28:21 AM »
Cletus, I watched Mr Pete's wobbler videos way back when, before I was hell bent on designing one. Thanks for the memory refresh. I'll have to go watch them again!

 

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